Foundations on extension with no load

[1234Ben]

Hi, I'm just commencing a number of projects on a house that we just completed on. More questions to come over the coming months.

First question relates to a small single storey flat roof extension that is infilling a step in across the back of the house - i.e. the house is flat across the back apart from a centre section which is a metre or so inset. This is 3.5m x 1m, and there won't be any new walls built, the outer will be bifold doors with the lintel bearing on the existing masonry walls to each side of the step in - i.e. the new slab does not have any load on it.

Does anyone have any experience of what foundations are needed - a full deep trench seems overkill given the floor is only supporting a set of bifold doors and one side of a 1m flat roof, but appreciate that may be needed to get a stable level to pour the slab. Any thoughts appreciated.

 

[freddiemercurystwin]

You could get an engineer to design a raft foundation if the ground conditions permit it, otherwise you would build a wall under the edge of the slab/bifolds, which would need a foundation.

 

[^woody^]

Check your construction knowledge! No floorslabs have any load on them, and they do not support external doors. Doors are supported off walls, on foundations or on lintels or encased beams.

 

[freddiemercurystwin]

Bifolds are typically floor supported or sometimes suspended off whatever's above, they're not suspended by the walls.

 

[^woody^]

Off a wall ..... on a foundation
Off a wall ..... on a lintel
Off a wall ..... on an encased beam

 

[Deluks]

What if some future owner decides that they want to remove the bi-folds and replace it with a wall and a single door, or just completely fill it in?

 

[^woody^]

What if some future owner decides that they want to remove the bi-folds and replace it with a wall and a single door, or just completely fill it in?

Who cares? Lol.

The OP can't plan for the whims of any future owner for any part of the building.

 

[1234Ben]

Thanks for the replies, I'm certainly not going to do anything inappropriate, and BC will inspect and approve anyway. The outer 'wall' will essentially be a lintel between the 2 existing walls at the sides so all the load from any roof structure will go through those existing side walls into the existing foundations. Underneath the lintel it'll just be the bifold doors so pretty minor - I suspect they are floor standing rather than suspended. Ultimately I appreciate we may need a traditional foundation if that's the right thing to do but given it'll only be supporting a few courses of blocks/bricks up to DPC level, and the edge of a small floor slab I wanted to look into whether there are other alternatives which are commonly used in these situations to reduce the need for deep excavation.

 

[cdbe]

Concrete lintel (about £400) just for outer skin with a few courses of brickwork on top and your slab butted up. Obviously clear with BC first.

 

[Deluks]

Who cares? Lol.

The OP can't plan for the whims of any future owner for any part of the building.

It just seems to make sense to me, that if you are extending the perimeter of a building, to build it to a standard that will allow for future alterations. A little bit extra work could save someone a massive ball ache down the line, and that person might be oneself.

 

[^woody^]

It just seems to make sense to me, that if you are extending the perimeter of a building, to build it to a standard that will allow for future alterations. A little bit extra work could save someone a massive ball ache down the line, and that person might be oneself.

The type of foundation, or lintel, or beam that will be acceptable to building control to take the small section of wall at ground level and the doors, would be sufficient to take any full or part walls in any future adaptation.

 

[cdbe]

He's a smart arse, but it does make sense - the roof is supported elsewhere so it's it's either the doors or a bit of infill to replace the doors.

 

[freddiemercurystwin]

No he thinks he's a smart arse but really he's dug himself into yet another hole, a foundation hole if you will, so will continue to come back with the witty quips that make people snigger yet have no substance ......

 

[^woody^]

There, there diddums, stay off the lemons you know what they are doing to you.